With the advancement of the information society and the spread of the Internet, communication capacity has increased exponentially alongside the ever-growing demand for large-capacity optical communication.
As part of the efforts to keep up with the increase in communication capacity, methods of increasing the speed of transmission of optical signals have been suggested. These methods, however, cannot increase signal transmission speed over the current maximum of 10 Gbps or 40 Gbps. In order to address this, wavelength division multiplexing (WDM) has been widely adopted. WDM is an optical technology that simultaneously transmits a number of wavelengths using a single optical fiber.
Due to the development of WDM systems which multiplex a plurality of wavelengths into a single optical fiber and transmit the result of the multiplexing, networks have evolved from point-to-point static networks in which signals can only be transmitted via fixed paths into ring networks or mesh networks which can be dynamically configured according to necessity.
As transmission capacity increases, optical/electrical/optical cross-connection systems at ring or mesh network nodes, which convert wavelength-division-multiplexed optical signals into electrical signals and perform electrical processing, become more likely to become an electrical information processing bottleneck, and electrical information processing costs increase accordingly.
A cross-connection system having a simple structure can be provided at low cost by performing optical-to-electrical (O/E) conversion or electrical-to-optical (E/O) conversion only on optical signals with wavelengths that are supposed to be added/dropped at each network node while allowing transmission of optical signals with wavelengths that are supposed not to be added/dropped at each network node.
In the meantime, in the case of transmission of signals without a requirement of optical/electrical/optical conversion, nearly all types of electrical signals can be converted into and transmitted as optical signals regardless of their frame formats and the speed of transmission channels.
However, a network using an optical/optical/optical cross-connection system, unlike a network using an optical/electrical/optical cross-connection system which generate new signals for each network node, does not regenerate optical signals for intermediate network nodes, thereby causing degradation of optical signals and imposing restrictions on transmission distance and network expandability.
Electrical interconnection/grooming switches, which can be used in an optical/electrical/optical cross-connection system, perform switching in units of low-speed electrical digital hierarchy signals. More specifically, since electrical interconnection/grooming switches have a grooming function by which optical signals are classified into high-speed optical signals and low-speed optical signals are reclassified according to their paths, and then combined, electrical interconnection/grooming switches can efficiently utilize bandwidths. On the contrary, optical switches, which can be used in an optical/optical/optical cross-connection system, handle high-speed signals and cannot efficiently utilize bandwidths because of their wide switching bandwidth.
In conclusion, a cross-connection system at each network node is required to minimize the number of O/E and E/O conversion operations and make an efficient use of bandwidths by passing therethrough optical signals that are supposed not to be added/dropped at a corresponding network node without converting them into electrical signals, performing O/E conversion only on optical signals that are supposed to be added/dropped at the corresponding network node, and grooming low-speed electrical digital hierarchy signals.
With the advent of Internet Protocol (IP) TV, Virtual Private Networks (VPNs), and Storage Area Networks (SAN), the demand for multicast technology for sending signals from one source to more than one destination at the same time has steadily increased.
Multicasting can reduce network traffic by integrating signals through common path into a single signal, transmitting the single signal by a predetermined distance, and enabling a branch network node to split the single signal and to transmit the split signal to each of a plurality of destinations, instead of creating a plurality of signals that are destined for different destinations and transmitting the signals separately.
Conventionally, electrical IP routers perform a multicast operation because they cannot provide a switching function in an optical communication network where IP signals are transmitted.
However, multicasting can be performed even in a physical layer by using an optical cross-connection system having an optical switching function and designing a network node to be able to perform a multicast function.
FIG. 1 illustrates a network node using a conventional optical/electrical/optical cross-connection system. Referring to FIG. 1, the network node includes a demultiplexer 100, an O/E converter 110, an electrical interconnection/grooming switch 120, an E/O converter 130, and a multiplexer 140.
In order for the optical/electrical/optical cross-connection system to switch paths, a wavelength-division-multiplexed signal is demultiplexed into a plurality of wavelengths by the demultiplexer 100, and the demultiplexed signal is converted into an electrical signal through O/E conversion performed by the O/E converter 110.
The electrical interconnection/grooming switch 120 recombines or adds/drops the electrical signal provided by the O/E converter 110. An electrical signal provided by the electric interconnection/grooming switch 120 is converted into an optical signal through E/O conversion performed by the E/O converter 130.
The optical signal provided by the E/O converter 130 is multiplexed by the multiplexer 140. Thereafter, the multiplexed optical signal is output to each port.
The optical/electrical/optical cross-connection system subjects all wavelength-division-multiplexed signals to O/E conversion and then E/O conversion. Thus, as transmission capacity increases, the optical/electrical/optical cross-connection system become more likely to become an electrical information processing bottleneck and increase electrical information costs.
The electrical interconnection/grooming switch 120 can copy electrical signals. Thus, a multicast operation may be performed using the electrical interconnection/grooming switch 120.
FIG. 2 illustrates a network node using a conventional optical/optical/optical cross-connection system. Referring to FIG. 2, the network node does not include an O/E converter and an E/O converter but includes an optical interconnection switch 240 which interconnects input and output ports.
In order to perform a multicast operation, a wavelength to be multicast may be set in advance and a signal may be copied into multiple paths by a coupler 230 and be switched to the multiple paths.
The network node illustrated in FIG. 2 has a simple structure because of the lack of an O/E converter. However, the network node illustrated in FIG. 2 cannot prevent degradation of optical signals. In addition, since the network node illustrated in FIG. 2 does not have a wavelength conversion function, the network node illustrated in FIG. 2 may cause a wavelength collision, thereby reducing the availability of network resources.
A network node using an optical/optical/optical cross-connection system (the net work node illustrated in FIG. 2) demultiplexes a multiplexed signal into a plurality of wavelengths and switches each of the wavelengths, thereby causing degradation of optical signals due to a filtering operation performed by an arrayed waveguide grating used for demultiplexing/multiplexing.
The network node illustrated in FIG. 2 determines in advance a wavelength to be multicast. Thus, as the number of output paths increases, the number of branches increases accordingly and thus, the network node illustrated in FIG. 2 is highly likely to result in loss of optical signals.
FIG. 3 illustrates a network node which is capable of increasing the availability of network resources by adding a wavelength converter to a conventional optical/optical/optical cross-connection system. The network node illustrated in FIG. 3 is the same as the network node illustrated in FIG. 2 except that it further includes a wavelength converter 310 and an optical interconnection switch 320. The network node illustrated in FIG. 3 can prevent the availability of network resources from being reduced by a wavelength collision.
However, the network node illustrated in FIG. 3 is inefficient because it requires an additional wavelength converter, suffers from the same problems as the network node illustrated in FIG. 2, and fails to utilize bandwidths due to its wide switching bandwidth.